U.S. patent number 4,781,944 [Application Number 06/831,409] was granted by the patent office on 1988-11-01 for process and apparatus for fixing, encapsulating, stabilizing and detoxifying heavy metals and the like in metal-containing sludges, soils, ash and similar materials.
Invention is credited to Bradford H. Jones.
United States Patent |
4,781,944 |
Jones |
November 1, 1988 |
Process and apparatus for fixing, encapsulating, stabilizing and
detoxifying heavy metals and the like in metal-containing sludges,
soils, ash and similar materials
Abstract
Heavy metals and compounds thereof and other toxic materials in
industrial wastes, sludges, soils, incinerated ashes and the like
are fixed and stablized in a char residue, obtained by critical
region pyrolyzing techniques and appropriate proportions of
cabonaceous materials intimately mixed with the sludge, to
encapsulate the heavy metals with carbon bonded thereto which
effectively detoxifies the residue and renders it immune to any
substantial leaching out or later exposure to the toxic metals,
such that the same is environmentally safe for such uses as
landfill and the like.
Inventors: |
Jones; Bradford H. (Stratham,
NH) |
Family
ID: |
25258985 |
Appl.
No.: |
06/831,409 |
Filed: |
February 20, 1986 |
Current U.S.
Class: |
427/228;
405/128.85; 405/129.28; 423/53; 427/399; 427/403; 427/419.7 |
Current CPC
Class: |
B09C
1/065 (20130101); C10B 53/00 (20130101) |
Current International
Class: |
B09C
1/00 (20060101); B09C 1/06 (20060101); C10B
53/00 (20060101); B05D 003/02 (); C01G 037/02 ();
C01G 037/14 () |
Field of
Search: |
;427/228,403,399,419.7
;405/129 ;423/53 ;118/61,69,64 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Bell; Janyce
Attorney, Agent or Firm: Rines and Rines, Shapiro and
Shapiro
Claims
What is claimed is:
1. A process for fixing and stabilizing heavy metals in a
metal-containing sludge, soil, or ash feed material that comprises,
heating the metal containing material with intimately contacting
carbon-containing material to a temperature sufficient to drive off
volatile organics and solvents, but below the temperatures for
complete volatilization of most heavy metals and while inhibiting
the formation of oxides of nitrogen and sulfur, and in a
substantially oxygen-free environment; continuing the heating for a
period of time sufficient to produce a char residue with carbon
bonded to the metal, the heating being continued until the weight
of the residue expressed as a percentage of the weight of the metal
containing material is a minimum and until substantially all
volatile organics and solvents are driven off; cooling the char
residue in the absence of air to a temperature of about 250.degree.
F. to ensure that the residue will no longer combust if exposed to
air; and thereafter cooling the residue further to ambient
temperature to provide a non-leachable residue.
2. A process as claimed in claim 1 and in which, after said cooling
to ambient temperature, water is added to prevent dusting.
3. A process as claimed in claim 1 and in which the cooled residue
is encapsulated in cement and the like, rendering the same
fire-proof.
4. A process as claimed in claim 1 and in which silicate and
clay-type materials are added to condition the residue and prevent
dusting of the cooled residue.
5. A process as claimed in claim 1 and in which the heating
temperature is of the order of 900.degree. F. and above.
6. A process as claimed in claim 1 and in which the volatilized
organics and solvents are further oxidized to destroy the same.
7. A process as claimed in claim 1 and in which said metal
containing material is pre-dried at least in part before said
heating in the oxygen-free environment.
8. A process as claimed in claim 7 and in which the said volatiles
driven off are after-burned and scrubbed before exhausting.
9. A process as claimed in claim 8 and in which the after-burned
volatiles are used to provide non-contact drying in the pre-drying
of said metal containing material.
10. A process as claimed in claim 8 and in which wet air from the
pre-drying of said metal containing material is condensed into dry
air and is combined with the said volatiles driven off for said
after-burning.
11. A process as claimed in claim 1 and in which said metal
containing material is substantially non-organic and is intimately
mixed with an added carbon-containing source before said
heating.
12. A process as claimed in claim 11 and in which said source
comprises a carbonaceous fuel.
13. A process as claimed in claim 11 and in which said source
comprises an organic sludge.
14. A process as claimed in claim 1 and in which the carbon bonded
to the metal forms a coating thereabout.
15. A process for fixing and stabilizing heavy metals in a
metal-containing sludge, soil, or ash feed material, that
comprises, heating the metal containing material with intimately
contacting carbon-containing material including an added solvent to
a temperature sufficient to drive off volatile organics and
solvents, but below the temperatures for complete volatilization of
most heavy metals and while inhibiting the formation of oxides of
nitrogen and sulfur, and in a substantially oxygen-free
environment; continuing the heating for a period of time sufficient
to produce a char residue with carbon bonded to the metal, the
heating being continued until the weight of the residue expressed
as a percentage of the weight of the metal containing material is a
minimum and until substantially all volatile organics and solvents
are driven off; cooling the char residue in the absence of air to a
temperature low enough to ensure that the residue will no longer
combust if exposed to air; and thereafter cooling the residue
further to ambient temperature to provide a non-leachable residue.
Description
The present invention relates to processes and apparatus for
detoxifying heavy metals and the like contained in sludges, soils,
incinerated ashes and similar materials; being more particularly
directed to novel techniques for effecting the encapsulation,
fixing and stabilizing of such residues to render them safe (by at
least Environmental Protection Agency (EPA) standards) from the
leaching out of, or later exposure to, heavy metal products
therein, such that the residues may be directly used in landfills
and other applications, if desired.
Previous approaches to the solution of the above problem have been
made through the use of chemical fixation and stabilization
techniques, including the combining of silicates and Portland
Cement to produce a stabilized, solidified material. While such
processes have some effectiveness in binding heavy metals, the
effectiveness in stabilizing volatile organics is questionable, and
the solidification process unfortunately requires increasing the
volume of the sludge by ten to twenty percent. The solidified
product is landfilled or may be used for landfill cover material
but, as compared with the later-described fixed product of the
present invention, is at least twice as expensive, with the
invention providing most advantageously for about eighty seven
percent less weight and seventy percent less volume in some
tests.
In accordance with the techniques employed to attain the results of
the present invention, a pyrolysis step is employed that of itself
has been used in prior systems that are and have as their
objectives the antithesis of the current invention; that is, the
deliberate recovery of the heavy metal from the end product. One
such is described, for example, in U.S. Pat. No. 4,332,584 where
the resulting carbon and heavy metal residue can be centifuged to
recover chromium, for example, and the chromium may be readily
leached out by dissolving in sulfuric acid solution. Other metal
recovery techniques, as distinguished from metal fixing techniques,
are disclosed in my prior U.S. Pat. No. 4,086,319 and in U.S. Pat.
No. 4,215,989.
Examples of materials with which the fixing, stabilizing and thus
effective detoxifyzing of heavy metals and the like of the
invention may be employed include sludges, wastes such as tannery
waste with its high chromium content, soils, ash materials and the
like (often hereinafter generically referred to as "sludges"); and
typical heavy metals or metal compounds (often hereinafter
generally referred to as "metals") include chromium, arsenic,
barium, cadmium, lead, mercury, selenium, silver, nickel, zinc,
copper and others.
In accordance with the present invention, unlike prior pyrolysis
systems, it has been discovered that the chrome or other heavy
metal sludge, tannery wastes or other heavy metal-containing
solutions and the like may be treated in such a manner that the
heavy metals are effectively non-leachable and cannot be recovered;
and, indeed, are fixed and encapsulated with carbon-bonded coatings
that enable the residue to be safely used for landfill and other
uses without fear of ultimate leaching out of, or other later
exposure to, the heavy metals from the end product. While the step
of pyrolysis or heating of the sludge or tannery waste or the like,
in a substantially oxygen-free atmosphere, is a step common to
other processes and the present invention, thus, the way in which
this step is utilized and subsequent steps are performed is
uniquely designed to obtain the very different and indeed opposite
result of a heavy metal-fixed char residue that is safe to the
environment and from which the heavy metals cannot effectively be
leached out.
An object of the present invention, accordingly, is to provide a
new and improved process and apparatus for fixing, stabilizing and
effectively detoxifying heavy metals in a metal-containing sludge,
soil, ash or the like that is not subject to the above-described
disadvantages of prior chemical fixing and other techniques, but
provides vastly improved, less costly and more efficacious
stabilizing not only of the resulting residue but of the volatile
organics and solvents driven off in the process, as well.
A further object is to provide such a new process in which a novel
intimate carbon-coating is bonded to or encapsulated with heavy
metal residues to render the same permanently substantially
non-leachable.
Other and further objects will be explained hereinafter and are
more particularly delineated in the appended claims.
In summary, however, the invention involves a volume reduction
process that thermally stabilizes the heavy metals and volatilizes
organic compounds. The resulting residue often has fifty to eighty
percent less volume than the original sludge or composite feed. The
process operates in a starved air combustion unit capable of
controlling temperature and residence time; and resulting residue
contains an acid-insoluble matrix. This process produces about
fifty percent more residue than incineration, but admirably passes
EPA's Extraction Procedure Tests for leaching, heavy metals, and is
therefore well-suited for landfill disposal. Heavy metals and other
potentially toxic compounds are stabilized, moreover, at
temperatures below their volatilization temperature, along with an
intimate mixture of organic compounds. The thermal residence time
is controlled to produce minimum volume without significant
decarbonization (the latter being causable by excess oxygen).
Toxic, volatile off-gas and odors are destroyed within a high
temperature afterburner designed for appropriate (two-second
minimum) contact time.
This process, as later discussed, has been tested effectively on
sewage treatment plant sludges, among other materials, containing
potentially toxic concentrations of chromium, lead, cadmium,
barium, nickel, zinc and other toxic heavy metals, with the
resulting residues tested using both the EPA Leaching Test
employing acetic acid to acidify the residue to a pH of 5, and the
well-known Multiple Extraction (ME) Test that utilizes a subsequent
sulfuric and nitric acid mixture to acidify the residue to a pH of
4.
While sludge incinerators and trash incinerators often produce ash
residues containing toxic, leachable metal oxides and dioxin, the
residue produced with the process of the present invention does not
contain significantly soluble metal oxides (e.g., chromium), as
later tabulated. The residue, furthermore, can be mixed at ten to
fifteen percent, for example, of Portland Cement (on a weight
basis), if desired, with a quantity of water equal to the weight of
the residue, and some coarse sand. The resulting cemented residue
material has a specific gravity of about one, it is fireproof, and
it has sufficient structural strength to be safely used for
lightweight building products.
Another illustrative application of the invention is for
stabilizing toxic metals within waste sludges from steel drum
reconditioning plants, the empty drums containing a variety of
residual materials which often contain toxic metallic and organic
compounds. The resulting residues show a 50-percent reduction in
volume with the toxic metals immobilized with carbon, and the
solvents destroyed in the afterburner. The invention can also be
used to process toxic waste and eliminate the need of disposing of
these materials in hazardous waste landfills; with the residue used
for landfill or solidified for use in building products such as
cinder blocks, asphalt, concrete or other lightweight products, as
before suggested.
As still another use, the invention may aid in the recovery of
chromium from the leather and metal plating industries since the
char residue passes the EPA Extraction Procedure, indicating that
the residue will not leach excessive quantities of chromium.
Subsequent roasting of such residues containing chromium produces a
chromate ash from which chromium can be extracted, as in some of
the above-cited patents, with the remaining insoluble residues
treated by the invention to produce a non-toxic residue.
When a carbon source is mixed with an ash containing toxic
chromium, as still another example, the process of the invention
involving starved air conditions, results in a residue that cannot
leach any significant levels of chromium, such that the invention
allows toxic ash to be effectively detoxified, as well.
Summarizing the technique underlying the invention, from one of its
important aspects, the same embodies a process for fixing and
stabilizing heavy metals in a metal-containing sludge, soil, ash
material and the like, that comprises, heating the sludge with
intimately contacting carbon-containing material to a temperature
sufficient to drive off volatile organics and solvents, but below
the volatilization temperature of the heavy metals and while
inhibiting the formation of oxides of nitrogen and sulfur, and in a
substantially oxygen-free environment; continuing the heating to
the point where a resulting char residue develops with carbon
bonded to the metal as a coating thereabout and from which
substantially all volatile organics and solvents are driven off, as
indicated by substantially reaching the minimum residue weight;
cooling the char residue in the absence of air to a temperature at
which it will no longer combust if exposed to air; thereafter
cooling further to ambient temperature to provide a non-leachable
residue; and using the same for landfill and the like. Preferred
process steps and best mode details and apparatus are hereinafter
presented.
The invention will now be described with reference to the
accompanying drawings,
FIG. 1 of which is an apparatus and flow diagram illustrating the
process and apparatus of the invention applied to the use of
organic feed materials (such as sludge, soils, hazardous waste,
incinerator ash and organic mixtures) as the fuel;
FIG. 2 is a graph illustrating the critical zone of operation
required for the novel results of the invention;
FIG. 3 is a flow diagram similar to FIG. 1 and FIG. 4 adds
predrying steps; and
FIGS. 5 and 6 are similar diagrams of the process adapted for
non-organic feed with externally applied carboneous fuel, and
FIGS. 7 and 8 add drying steps.
Referring to FIG. 1, an apparatus for practicing the process of the
invention as applied to the detoxifying (by fixing and stabilizing
as before described) of industrial waste sludges including chromium
as from tannery waste and the like, is illustrated.
Sludge containing toxic heavy metals and volatiles is de-watered on
a belt press 2, and is conveyed to a tumble dryer 4 (as of the
Tumbler Dryer Type of Bartlett Snow Pacific Co., for example). This
dries the feed material from the one-third solids coming from the
sludge belt press 2 to approximately 65% solids, utilizing waste
heat generated by the afterburner 6, to be described later. The
feed material dried to 65% solids then enters at chute 4' (by
gravity) into a pyrolysis unit 8, the size of which can be reduced
because of the effect of weight reduction caused by the at least
partial drying or pre-drying operation. The unit 8 provides a
largely airtight oxygen-free combustion furnace environment for
pyrolysis at temperatures, in this particular case, of around 900
degrees Fahrenheit, below the volatilization temperature of most
metals before mentioned (and, also, while inhibiting the formation
of oxides of nitrogen and sulfur as well, to avoid their presence)
but sufficient to drive off the volatile organics and solvents as
off-gases, later described. The critical conditions of operation
for attaining the fixings of the invention by carbon-bonding or
encapsulating, will later be described; a suitable pyrolysis unit
adjustable for such purposes being, for example, that of Shirco
Infrared Company of Dallas, Texas.
The before-mentioned afterburner 6 (also, for example, of the
Shirco Type) further oxidizes and volatilizes the off-gases and
volatiles at about 1800 degrees Fahrenheit; though if highly toxic
wastes are used, the afterburner temperature may be raised to
around 2400 degrees Fahrenheit. As earlier stated, the off-gases or
heat from the afterburner 6, in such case, may be used as at 6' to
dry the incoming sludge into the dryer 4. As illustrated,
non-contact drying is used so that the hot gases do not come in
contact directly with the incoming sludge. The hot gases and water
(entered into a venturi), are intermittently mixed and applied to a
wet scrubber system 18. Since there is a potential of hexavalent
chrome being generated in small quantities in the afterburner, a
reducing chemical is fed to reduce any hexavalent chrome to the
trivalent state. The final unit is a stack 10 which exhausts the
clean effluent gases to the atmosphere.
Assuming the carbon-encapsulating conditions, later described, in
the pyrolysis chamber 8, at the bottom of FIG. 1 there is shown the
ash removal system embodying an ash-cooling conveyor 12 having an
airtight connection between the furnace 8 and the ultimate truck T
going to the landfill. The ash is thereby watercooled to reduce the
temperature of the material from 900 degrees Fahrenheit to
somewhere below 250 degrees Fahrenheit at which exposure to the air
cannot result in combustion.
Returning, now, to the features of the process necessary to obtain
its novel results, underlying the invention is the discovery that
there is a criticality to the pyrolyzing or heating process in the
absence of oxygen in the furnace 8 that brings the process to a
point where the carbon-containing material in the sludge or the
like, becomes bonded to and encapsulated with the dangerous heavy
metal content.
This has been discovered to reside in the region where
substantially all the volatile organics and solvents in the
original sludge have been driven off at 6" in the substantially
oxygen-free atmosphere, whereby the resulting char residue is at a
point of substantially minimum weight. This minimum weight region
is shown at the shaded region "Preferred Stabilization Zone" of the
graph of FIG. 2, plotting percent of original weight of the sludge
applied at 4' along the ordinate as a function of time of pyrolysis
plotted along the abscissa in minutes for the approximate
900.degree. F. of the above example. In this test it took about 24
minutes to pyrolize the carbon-coated material to its minimum
weight of about 8-9%. This is as distinguished from operation on
the left-hand steep curve regions of prior art metal recovery, as
contrasted from the fixing result of the present invention.
At such a time, the char residue, again in the absence of air, is
cooled down at 12 to a temperature where it will not ignite if
exposed to air (about 250.degree. F. or less). Thereafter, cooling
continues to ambient temperature. Under such conditions, it is
found that the carbon is bonded or fixed to the heavy metal residue
in a state where is is substantially non-leachable and where is it
readily usable with safety for each purposes as landfill and the
like. To prevent dusting, water may be added in the utilization of
the residue product and/or it may be encapsulated in cement or the
like for producing fireproof materials, as before explained.
Alternatively, silicate and clay-type materials may be added to the
sludge to result in a non-dusting cooled residue.
It is important, however, that there be sufficient carbon material
in intimate contact in the sludge or other material to enable this
complete bonding, encapsulation, fixing and stabilizing of the
heavy metals in the residue, and this may be provided in solid form
or from sludges and liquids. Hydrocarbon solvents may be added to
the solid materials, as well.
The following Table 1 presents experimentally obtained results for
the above-described industrial sludge, demonstrating the
insignificant leaching of heavy metal residual contents by EPA's
Leaching Test procedure (center column, in mg/L) in the char
residue produced by the process of the invention, as compared with
the allowable (and much greater) EPA limits.
TABLE 1 ______________________________________ CHEMICAL
CHARACTERISTICS OF INVENTION RESIDUES BULK ASSAY MAXIMUM CHAR EPA
ACTUAL EPA RESIDUE LEACHATE* LEACHATE METAL (mg/L) (mg/L) (mg/L)
______________________________________ ARSENIC <0.05 5 BARIUM
1,633 20 100 CADMIUM 42 0.08 1 CHROMIUM 2,366 0.04 5 LEAD 200 0.48
5 MERCURY 0.23 <0.002 0.2 SELENIUM <0.1 1.0 SILVER 3.63
<0.005 5 ______________________________________ NOTE: *EPA's
Extraction Procedure Test Results.
As another example, Table 2 shows similar substantially
non-leachable or fixed conditions in a residue prepared by the
above process from a second POTW sludge with a high percentage of
industrial contribution:
TABLE 2 ______________________________________ BULK ASSAY SLUDGE
INVENTION MAXIMUM SOLIDS CHAR EPA mg/kgm DRY RESIDUE LIMITS METAL
WEIGHT mg/L mg/L ______________________________________ ARSENIC 8
<0.01 5 BARIUM 1,030 20 100 CADMIUM 309 0.27 1 CHROMIUM 1,490
0.04 5 LEAD 126 0.48 5 MERCURY 0.1 <0.002 0.2 SELENIUM <0.7
0.011 1 SILVER 28 <0.005 5 NICKEL 910 2.2 ZINC 2,210 1.0 COPPER
4,400 1.3 ______________________________________
Cost comparisons using waste water treatment plant dewatered
sludge, moreover, indicate that the present invention, with residue
disposal, costs from $30 to $40 per ton; landfill, about $50 to $60
per ton, and compost (assuming adequate controls on the
concentration of toxic industrial waste contamination), about $35
to $45, after deducting the value of the compost.
As still a further example of the efficacy of the invention, Table
3 presents the percent fixation achievable by the process of the
invention on a sample of treated wet inorganic sludge (chromium
hydroxide) at 19% Cr on a dry solids basis:
TABLE 3 ______________________________________ PERCENT FIXATION OF
TOTAL METAL IN RESIDUE EP TOXICITY 1 M SULFURIC PARAMETER
EXTRACTION ACID EXTRACTION ______________________________________
pH 5.0 <0.5 Cr, mg/l 0.01 64 Fixation, % 99.9999 99.5
______________________________________
A further test of the efficacy of the process underlying the
invention was made on both organic and inorganic sludges
(Cr(OH).sub.3) and the latter both in the presence of insufficient
carbon for adequate intimate admixing with the heavy metal sludge
to effect the total fixing results of the invention (as when so
moist as to cause a degree of decarbonization) and in the presence
of sufficient carbon, adequately contacted and intermixed, under
EPA Extraction Procedure (0.5N Acetic Acid at pH 5) and under
severe 1 Molar sulfuric acid extraction or leaching (20 times the
gram weight):
______________________________________ Total Chromium In Leachate
(mg/L) EP Leach Severe Sulfuric Acid
______________________________________ Sample A 0.01 (99.99%
fixation) Treated with IM Solution (Wet inorganic- 64 (99.5%
fixation) adequate hydro- carbon solvent) Sample B 0.065 1300 (Wet
inorganic with insuffi- cient carbon) Sample C 0.22 140 (Dry
inorganic) Sample D <0.005 72 (Dry organic)
______________________________________
These results again illustrate the importance of a sufficient
carbon source, either already present in the organic material
(tanning and other industrial wastes, gasoline-contaminated soil,
sewage, and other toxic wastes) or added to intimately mix with the
metal-containing sludge (such as added carbonaceous fuel, diesel,
gas, coal, etc,) to establish the results of the invention. In some
cases, moreover, it is preferable not to dry the residue to the
almost dry (>95%, for example) state used in heavy metal
recovery techniques, as for recovering chromium from char ash; but
rather, to limit the drying to keep sufficient moisture (such as in
the 65% solids earlier described, as an illustration) to insure the
intimate and adequate encasing of the metal-containing sludge or
ash or the like for the fixation purposes of the invention. The
invention also is a one-step process as further distinguished from
the antithetical recovery systems involving first oxidation and
then reduction.
Turning, now, to the process flow diagram of FIG. 3 that summarizes
the system of FIG. 1, the organic feed fuel (municipal sludge,
soils, hazardous waste, incineration ash and organic mixture, etc.
before discussed) is shown directly pyrolized under the critical
conditions of FIG. 2, before discussed, in the controlled-air
thermo-processor furnace 8, with off-gas generation for the
afterburner 6, mixed with incoming air for combustion in the
afterburner 6, and with the air-stream sent to scrubbers 18. The
metal-fixed non-toxic char residue from the processor 8 is
discharged after cooling in the manner 12 of FIG. 1. In FIG. 4, the
organic feed system of FIG. 3 is shown employing a pre-drying step
as in FIG. 1 in which the sludge is fed to the dryer 4 before
reaching the processor furnace 8, with the previously described off
gases from the afterburner 6 sent for non-contact drying as in FIG.
1. Wet air from the drier 4 is shown condensed to supply dry air
for mixing with the processor off-gases.
A further option, as before discussed, is to handle non-organic
sludge, soil and hazardous waste, as diagrammed in the flow process
of FIG. 5. In such a system there is opportunity to mix this feed
material at 16 with a hydrocarbon fuel (liquid or solid) first to
provide a direct mixing contact of the carbonless feed with a
sufficient amount of carbon for achieving the fixation process of
the invention. These soils can also be used as feed stuff as well
as any hazardous waste that may be necessarily required to be
detoxified. If the mixing at 16 involves solids, various
configurations, such as a pug mill, may be used for the solid/solid
intimate mixing. Again, the thermo-processor off-gases, mixed with
air, are applied to the afterburner 6 as in FIG. 3, and the stream
goes to scrubbing for emission control at the stack 10, and with
the fixed char residue discharging to the ambient air after cooling
at 12.
In the modified flow diagram of FIG. 6, on the otherhand, also
involving non-organic feed material, the same is fired with
carbonaceous fuel but, in this case, a gaseous or a liquid fuel is
added directly at 8' to the process furnace 8. Since the processing
of the invention at 8 is under starved air pyrolysis conditions, it
presents an option of having the feed material intimately joined
with the available carbon in a gaseous state during
thermo-fixation; and it would be used, in such cases, where the
nature of the feed material would allow for a gaseous carbon source
that would generate a char residue demonstrating the fixation
properties of the invention.
The system of FIG. 7 combines the pre-drying of FIG. 4 with the
mixing (16) of non-organic feed material with carbonous fuel of
FIG. 6. In the modification of FIG. 8, on the other hand, the
pre-drying (4) of FIG. 7 is combined with the application of
carbonaceous fuel at 8' into the processor pyrolysis furnace 8 of
FIG. 6.
As before mentioned, the dried metal-fixed char residue cooled at
12 and ultimately at ambient temperature, may have water added to
prevent dusting, or may be encapsulated in cement, or, indeed,
silicate and clay-type materials may have been added to prevent an
ultimate product that can dust.
Further modifications will also occur to those skilled in this art,
and such are considered to fall within the spirit and scope of the
invention as defined in the appended claims.
* * * * *